Diamond is widely used in sawing, drilling, dressing, and grinding applications. The diamond is typically bonded to or mechanically held in a matrix of nickel, copper, iron, cobalt, or tin, or alloys thereof which is connected to a tool body. The matrix can also comprise a resin, such as phenol formaldehyde.
When the diamond is in the form of an abrasive grit, it is frequently mechanically bonded in the matrix with the matrix surrounding the grit and holding it in place. While simple and practical, mechanical bonds are relatively weak and the grit can be easily lost as the surrounding matrix is abraded away during use. Grit retention can be improved by embedding the grit deeply into the matrix leaving little grit exposure, but this decreases cutting efficiency. In a typical saw blade application, the average exposure of diamond grit is less than 20% of the total grit height. Grit loss can become a serious problem when the supporting matrix is worn down such that over one-third of the grit is exposed. The average lifetime of such cutting tools is decreased as up to two-thirds of the original diamond grit are prematurely lost.
From U.S. Pat. No. 3,871,840, it is known that a tungsten coating deposited on a diamond particle enhances the ability of a matrix material, such as bronze or a bronze alloy, to adhere to the diamond particle and thereby to retain the particle in a tool during the use of the tool in abrading, cutting or grinding use.
Additionally, in attempts to improve grit retention, it has been known to coat diamond particles with carbide forming transition metals such as titanium or zirconium. The coating's inner surface forms a carbide with the diamond. A second layer of a less oxidizable metal, such as nickel or copper, can then be applied to protect the inner layer from oxidation.
Tensile testing of double layer coated diamond having an inner layer such as chromium and an outer layer such as nickel shows that fracturing occurs at the interface between the inner and outer metal layers. This suggests that nickel does not alloy or otherwise bond well with the underlying carbide and that double layer coated grits may not significantly improve overall grit retention. Bonding can also be weakened by oxidation of the inner chromium layer during the nickel coating process.
It is also known to coat diamond particles with titanium, manganese, chromium, vanadium, tungsten, molybdenum or niobium by metal vapor deposition. It has been found, however, that these carbide formers do not bond strongly enough to the diamond crystals to improve their grit retention for many high stress applications, or they are susceptible to oxidation. As discussed above, the outer metal layers used to protect inner layers from oxidation do not adequately bond to the inner layer.
As illustrated in copending, commonly owned U.S. patent application Ser. No. 07/261,236, the disclosure of which is hereby incorporated by reference, attempts have been made to further improve the bonding strength between a matrix and diamond particles by first coating the diamond particles with chromium and then depositing a coating of tungsten on the chromium by a chemical vapor deposition technique. A typical CVD technique is to levitate or tumble the chrome coated diamond in a vapor. The vapor is typically composed of tungsten hexafluoride. It has been discovered however, that while favorable results have been achieved by this process, there is an unfortunate tendency for the fluorine containing gas to react detrimentally with the chromium containing coating to produce chromium fluoride. Therefore, for CVD depositions utilizing tungsten hexafluoride, the strength of the layered system is limited by the presence of the chromium fluoride between the chromium carbide and the tungsten layer.